The present invention relates to a device at a knee joint prosthesis of the kind that comprises a polycentric link mechanism in accordance with the preamble of patent claim 1, as well as to an artificial knee joint in accordance with the preamble of the claims 4 and 10.
Knee joint prostheses may be divided into two main categories:
1) The knee joint prostheses which lock at heel strike;
2) The knee joint prostheses that do not lock at heel strike.
The latter type will not be described further.
The former type may be further divided into different types, which at heel strike and during normal walking on a planar surface functions as follows:
a) Manual locking prior to walking. Completely mechanical lock that does not permit any bending or shock absorption. Manual locking shoulder or piston being manoeuvred with an actuating lever or wire. Very simple technique where the prosthesis wearer walks with a straight leg and unlocks the knee before sitting down.
b) Automatic locking at heel strike. Completely mechanical lock, which cannot be bent at all, and do not show any shock absorbing capacity. The lock may for instance be in the form of a frictional lock, when is activated by the body weight.
c) Automatic locking at heel strike. It is possible to bend 10-15xc2x0 with resistance and shock absorption, however it is not resilient, for instance hydraulics.
d) Automatic lock at heel strike. It is possible to bend 10-15xc2x0 with shock absorption and resistance from an elastic element, which springs back to an extended position.
The walking pattern of a person having an amputated thighbone considerably differs from normal walking due to the absence of important muscle groups. By means of the loss of the musculature of the knee, the person can no longer bend and then actively stretch the knee joint during load. It has earlier been attempted with assistance of different braking mechanisms, with more or less reliability, to prevent the leg from collapsing during the supporting phase, i.e., the time period that the foot is in contact with the supporting surface. Thus, different degrees of stability have been obtained. The amputated person puts down the prosthetic leg with a straight knee, which then remains straight during the major part of the support phase. If the mechanism of the knee joint is very stable, it is not possible to bend the knee until the support phase is completed. The absence of knee flexions in the beginning and end of the support phase makes the walk of a person with an amputated thighbone more energy demanding. The absence of calf musculature also leads to prosthetic walking that is more energy demanding than ordinary walking.
Knee joint prostheses having polycentric link mechanisms are previously known, such term relates to the fact that the center of rotation has different positions at different angels of the knee joint, i.e., there are multiple joint centers. The human knee joint is also polycentric. An example of a polycentric knee joint is shown in U.S. Pat. No. 2,638,605, where the locking of the knee joint at a straight knee is accomplished by a catch, that is released by a link mechanism, which transfers the movement of the toe region to the knee joint. However, this constructional solution is very complicated, and thereby suffers from the great disadvantage of making the knee joint heavy. Furthermore, it does not provide any natural finishing of the walking phase.
In WO 91/15170 a knee joint prosthesis is described, which is constructed in such a way as to resemble the function of the human knee joint, which in turn results in a more natural walk of the amputated person. The knee joint comprises a four-jointed polycentric link mechanism, in which the thigh bone prosthesis and the shank prosthesis are coupled by means of two links, one front and one back link, both jointed at either end to the thigh bone prosthesis as well as the shank prosthesis. The base link is of different length and attached to the shank prosthesis at different heights. The jointed attachment points of the shank prosthesis are arranged in mobile attachment elements, which, at a substantially straight knee joint, contribute so that the knee joint is in a substantially locked or mobile position, depending on the direction of the load on the knee joint. A mechanical lock stop, which may eventually be made from an elastically compliant material, is arranged to limit the forward movement of the back base link and its attachment elements as the knee joint is in a substantially straight position. This leads to the fact that the knee joint always is locked as long as the weight rests on the heel, and it cannot be unlocked even if considerable forces are applied to bend the joint. However, with assistance from the elastically compliant material in the mechanical lock stop the knee joint can be bent about 10-15xc2x0 in a locked position with rebound. This resembles a normal walk and compensates the absence of the calf muscle during an ordinary walking. This cushioned slight outward flexing during normal walking is normally called stance flex. Thus, with stance flex in the following is intended an angle between the thigh bone-device and the shank device, which usually amounts to between 5xc2x0 and 15xc2x0.
A first object with the invention is to provide a knee joint, which admits the possibility to a controlled bending of the knee to an angle exceeding the stance flex at load on the heel by means of that the moment of flexure is taken up by a resiliency device at the same time as springs back from this resiliency device is avoided at the swing phase of the leg. This object is achieved according to the invention by means of artificial knee joints in accordance with the characterizing part of claims 4 and 10.
A further object with the invention is to provide a knee joint that admits a controlled flexing of the knee up to an angle exceeding 20xc2x0, preferably at least 90xc2x0 at load on the heel, by means of that moments of flexure are taken up by a resilient device, at the same time as spring from this resilient device is avoided during the swing phase of the leg. This object is also achieved, in accordance with the invention by means of artificial knee joints according to the characterizing part of claims 4 and 10.
A further object with the present invention is to achieve a farther development of the knee joint prosthesis mentioned above, where the stop function has been modified in such a way that it is possible to flex the knee joint at a higher degree with rebounding resistance as the load is on the heel, which is desirable, for instance, at walking in stairs and on inclined surfaces, as well as during practicing of certain sports, in order to obtain a natural pattern of walking and ability to move. This has been solved, according to the invention by means of the fact that the element of attachment shows a curved edge or similar, arranged to cooperate with the resilient stop device in such a way as to admit an increased mobility forward of the attachment element, and the back base element during compression of said resilient stop device, whereby flexing of the knee joint is admitted under load in a locked position.
The objective of the new modified stop function is to as large extent as possible to compensate for the lack of a working extensor musculature.
According to one embodiment, the curved edge or similar of the attachment device is arranged to act upon the resilient stop device via a dampening device. This may be constituted of a hydraulic system with an adjustable flow resistance. Hereby, the resistance at bending and active movement towards extension of the knee joint in a locked position may be adjusted for an individual adjustment. Hydraulic resistance, as well as spring back, may be individually adjusted.